PALEOSOL FROM THE 3.7 GA ISUA GREENSTONE BELT, SOUTHWEST GREENLAND

A lens of black schist within 3.7 Ga quartzites of Greenland may be Earth’s oldest known paleosol. The putative paleosol is a bed of berthierine schist with large ripidolite poikiloblasts. It can be interpreted as a saponite clay soil with a salt-rich horizon of kieserite, similar to other acid-sulfate paleosols of the early Earth. It is deeply weathered and forms the culmination of a trend of chemical weathering seen in other metasediments of the Isukasia area. It also has the field appearance, crack fills, clods, desert roses, textural profile, mineral composition, molar ratio variations, and REE depletion of a well drained paleosol. Models for proton and electron consumption of soils applied to the putative paleosol reveal an atmosphere with only 36 ± 510 ppm O₂ and 820 ± 1 ppm CO₂, and humid, cool temperate paleoclimate. The paleosol profile has organic δ¹³C_PDB consistently of -24.2 to -27.4 ‰, and modest Raleigh distillation up-profile. Similar consistent values and trends are found in modern living soils. In contrast, carbon isotopic composition of sediments is erratic from bed to bed, and meteorite carbon isotopic composition differs dramatically for each kerogen particle. Thus life in this very ancient soil is not precluded by our analyses.